A semiconductor nanoparticle with a particle size of 10 nm or less is in the state of a transition region between a bulk semiconductor crystal and a molecule, so that it has physicochemical properties different from both of them. In such a region, orbits are discrete because degeneration of the energy band seen in a bulk semiconductor is released and a quantum size effect appears in which the energy spread of the forbidden band changes depending on the particle size. According to the appearance of the quantum size effect, the energy spread of the forbidden band of the semiconductor nanoparticle decreases or increases depending on increasing or decreasing the particle size. The change of this energy spread of the forbidden band affects the fluorescence properties of the particles. One nanopartical has a smaller particle size and wider energy spread of the forbidden band has a fluorescent wavelength on the shorter wavelength side, and one nanopartical has a larger particle size and a narrower energy spread of the forbidden band has a fluorescent wavelength on the longer wavelength side. That is, it is possible to create a desired fluorescent color by controlling the particle size. The semiconductor nanoparticle also has high durability against an excitation light, etc., a region which can be excited widely spreads on the shorter wavelength than the fluorescent wavelength. Simultaneous excitation of multiple fluorescent colors is also possible by using a single excitation light source, resulting in this receiving attention as a fluorescent material. Specifically, the fields related to biotechnology (Nature Biotech. 21:41 (2003), etc.) and to optical device technology (Nature. 420:800 (2002), etc.) are listed as a field in which it has been used aggressively, and further applications in the future are expected.
In order to use the semiconductor nanoparticle as a fluorescence material, it is desirable that it has fluorescence properties in which the fluorescence spectrum has a waveform with a narrow and sharp full width half maximum. This is necessary in order for it to make use of the band gap fluorescence properties due to the forbidden band spread of the semiconductor nanoparticle. However, even if the particle size of the prepared bulk particles is made as a monodispersion, the band gap fluorescence properties are not sufficiently to manifest as is. As such, the presence of the energy level existing mainly at the surface site of the semiconductor nanoparticle is mentioned. Since the energy level in question exists in the forbidden band inside the particle, it has been thought that the band gap fluorescence properties are inhibited. From the reasons mentioned above, it has become a big subject that the aforementioned energy level is made inactive and that the band gap fluorescence is brought out.
As a method for bringing a solution to this subject, a (CdSe) ZnS nanoparticle which has a so-called core-shell type structure has been put forward. The aforementioned method is one in which the band gap fluorescence properties are made effective by coating the semiconductor nanoparticle (CdSe) with a second semiconductor material (ZnS) which has a larger forbidden band spread compared with the particle in question and eliminating the energy level in the forbidden band of the particle in question, resulting in high luminescence properties being obtained (JP-A No. 523758/2001 and J. Phys. Chem. B. 101:9463 (1997)). Additionally, CdS is coated on the CdSe nanoparticle as described in J. Phys. Chem. 100:8927 (1996) and ZnS is coated on the CdS nanoparticle as described in J. Phys. Chem. 92:6320 (1988). However, in any of these instances, there was a problem that a high temperature reaction was necessary using a highly toxic reagent or that commercially adequate fluorescence properties cannot be obtained.
On the other hand, inventors have been studying a method for making band gap fluorescence by making a monodispersion of the particle size in an aqueous solution and improving the particle surface. As a method for making band gap fluorescence properties in an aqueous solution, a method described in J. Am. Chem. Soc. 109:5649 (1987) is well-known. However, any method based on the method in question could not bring adequate fluorescence properties. As a result of studies carried out earnestly by the inventors, a method to obtain commercially adequate fluorescence properties could be developed, in which semiconductor nanoparticles synthesized by a size-selecting photoetching technique are treated in a refining process, the surface of the particles reformed by sodium hydroxide or an amine-ammonium compound, the energy level at the particle surface made inactive by arranging the electron donor groups at the surface in question, and the band gap fluorescence properties made effective (JP-A Nos. 51863/2004 and 243507/2004, etc.) According to this method, synthesis of semiconductor nanoparticles which have high luminescence properties was made possible by using a safe and simple technique in an aqueous solution. Moreover, by coating an organic material thereon, we succeeded in obtaining a semiconductor nanoparticle having excellent light durability in an organic solvent (JP-A No. 103746/2005, etc.). Moreover, it has also been suggested that it is possible to functionalize the surface depending on the kind of compound. However, since problems still remain on the side of the chemical durability against pH etc. in an aqueous solution, a further improvement in technology has been desired.
As a method for solving the above-mentioned problems, a method for coating by various compounds can be contemplated. For instance, a method described in J. Am. Chem. Soc. 115:8706 (1993) is known as a method for coating with TOPO. However, JP-A No. 523758/2001 points out that it cannot be used commercially.
A method described in J. Am. Chem. Soc. 125:320 (2003) is known as an example to coat using an amphiphilic polymer. However, it is not similar to a nanoparticle of the present invention because it uses a semiconductor nanoparticle having a core-shell type structure modified by a thiol compound, and the original purpose of coating using the amphiphilic polymer in question is not to maintain the fluorescence properties.
A method described in Science, 298, 1759, 29 Nov. 2002 is known as an example to coat using phospholipids. However, it is not similar to a nanoparticle of the present invention because it uses a semiconductor nanoparticle having a core-shell type structure, and the original purpose of coating using the phospholipids in question is not to maintain the fluorescence properties.
A method described in JP-A No. 525394/2002 is known as a method for coating using a surface-active agent. However, it is not similar to a nanoparticle of the present invention because it uses a semiconductor nanoparticle having a core-shell type structure, and the original purpose of coating using the surface-active agent in question is to dissolve it in water and not to maintain the fluorescence properties.
A method described in Chem. Lett. 33 840 (2004) is known as a method for coating using polyethylene glycol, but a thiol compound is used for a modulator, so that it is not similar to the present invention.
There are various reports and applications in addition to the above-mentioned examples (J. Phys. Chem. 100:8927 (1996) and J. Phys. Chem. 92:6320 (1988)), but they are not similar to the present invention from the point of having a core-shell type structure, etc. or they do not one have luminescence properties which can be used for commercial applications.
As a method for controlling the particle size which can be suitable for the present invention, a size selective photoetching technique described in J. Phys. Chem. B. 105:6838 (2001), etc. can be enumerated. Here, a size selective photoetching technique will be described. Physicochemical properties of the semiconductor nanoparticle appear depending on the particle size due to the quantum size effect. Therefore, the properties are averaged in this state and it is not possible to adequately display the properties of the semiconductor nanoparticle. Therefore, it is necessary to carry out, using a chemical technique, a precise particle size separation of the semiconductor nanoparticles, which have a wide particle size distribution right after preparation, and to make a monodispersion by separating and extracting semiconductor nanoparticles having only a specified particle size. As a method to do the abovementioned operation, a size selective photoetching technique is enumerated. The size selective photoetching technique uses the fact that the energy gap of the semiconductor nanoparticle increases with decreasing particle size due to the quantum size effect and a metallic chalcogenide semiconductor is oxidized/dissolved by irradiating light under the presence of dissolved oxygen. It is a method for controlling the particle size to a smaller semiconductor nanoparticle by irradiating a monochromatic light, which has a shorter wavelength than the absorption edge thereof, to the semiconductor nanoparticles having a wide particle size distribution to make only semiconductor nanoparticle having large particle size photo-excite selectively and letting them dissolve. According to this method, in the case when light with a wavelength of 476.5 nm is irradiated, the particle size distribution of the semiconductor nanoparticle becomes an average particle size of 3.2 nm with a standard deviation of 0.19 nm. It exhibits a very narrow particle size distribution in which the standard deviation is about 6% of the average particle size. That is, an extremely monodisperse semiconductor nanoparticle solution can be obtained.